Method of determining connate water content of cores



Sept. 28, 1943. c, L E 2,330,721

METHOD OF DETERMINING CONNATE WATER CONTENT OF CORES Filed May 18, 194230 Cm. of Mercury ATTORNEY 5/, l00% ,Per cent suturofion'woier BY7724124 INVENTOR.

Patented Sept. 28, 1943 WATER CONTENT OF CORES Miles 0. Leverett,Houston, Tex., assignor to Standard Oil Development Company, acorporation of Delaware Application May 18, 1942, Serial No. 443,544

6 Claims.

The present invention is directed to a method of determining the connatewater content of cores or samples of subsurface formations.

It is essential to determine the connate water content of producingformations in order to determine the petroleum reserves therein. Byconnate water is meant that water which is found associated with therock in a petroleum reservoir 'at the time of first tapping by thedrill.

The present methods of determination of connate water content do notadequately take into account the fact that the cores frequently areflushed by water from the drilling fluid. Further, cores taken by anymethod may lose part of their liquid contents by evaporation prior tolaboratory examination. Both these features of conventional coreanalysis methods are overcome by employing the method of the presventinvention.

Other objects and advantages of the present invention may be seen from areading of the following description taken in conjunction. with thedrawing in which- Figs. 1 and 2 are diagrammatic illustrations ofapparatus suitable for use when carrying out the determination ofconnate water in accordance with the present invention; and

Fig. 3 is a curve indicating the relationship between the capillarypressure and per cent of saturation of a core from a typical producingformation.

It is postulated that in a petroleum containing reservoir superimposedover a water stratum, the connate water of the petroliferous parts ofthe reservoir is in equilibrium with the water in the underlying waterstratum. It can be shown that, in order for this to be true, the connatewater within the petroliferous parts of the reservoir must exist under apressure which is less than that of the water in the water table; thatis, the water immediately underlying the petroleum bearing sections. Itnow has been found that the amount of water retained by a core sample atequilibrium depends on this pressure deficiency or capillary pressure.

It is therefore proposed that any core sample which has not in some waybeen irreversibly altered since removal from the reservoir, can bereturned to the water content which it bore in the reservoir by bringingit again into equilibrium with the water-at a capillary pressureequivalent to that under which itexisted in the reservoir. Thiscapillary pressure can be evaluated from the fact that it is equal tothe product:

G-A -H where G is gravitational constant, delta rho is the difference indensity between the water and the non-aqueous fluid in the reservoir,and H is the difierence in elevation of the core in its originalpositionin the reservoir and that of the water table.

One form of apparatus which has been found suitable for practicing thepresent invention is that shown in Fig. 1. It will be understood thatthis is merelya diagrammatic illustration, and

5 that the actual apparatus employed in the laboratory will haveconventional refinements. In the drawing, numeral II is a wide-mouthedthistle tube, to the lower end of which has been attached a flexibletube l2 and leveling bulb I3. Mercury i placed in the apparatus so thatitwill fill a portion of the leveling bulb, the amount of mercuryemployed being such that a difference in pressure of approximately 30centimeters may be obtained in the enlarged portion of th thistle tube,as will be explained hereafter.

In the enlarged portion of the thistle tube there is a porous membraneof a fine granular solid, such as barium sulfate, saturated with asolution having approximately the same salinity as the water in the wellfrom which the core was taken.

This membrane is preferably formed by arranging a fritted glass plate [4in the thistle-tube, placing saline solution therein, and thendepositing a layer [5 of the granular barium sulfate on the frittedglass plate.

To test a core, the level of the saline solution is pulled to the levelof the top of the porous membrane I5 by lowering the leveling bulb, carebeing taken that the suction applied does not exceed the amount whichthe porous membrane i5 is capable of sustaining. The sample i6 is thenplaced in the thistle tube with its lower surface resting on the uppersurface of the membrane l5. Either before or after the sample has beenplaced, the leveling bulb I3 is positioned so that the diiference inpressure above and below the porous membrane is equivalent to thecapillary pressure under which the core existed in the reservoir.Preferably, the water used should have the same surface tension as theinterfacial ten sion of the water and petroleum in the reservoir, butthis I have found to be not absolutely essential in many cases. When theleveling bulb has been so arranged, a loose cover I! is placed on top ofthe thistle tube to prevent the undue loss of water by evaporation, andthe apparatus is allowed to stand in this position for a suitableperiod, such as several days, in order to allow the moisture content ofthe sample to com to equilibrium with the saline solution. The sample isthen removed from the apparatus, weighed, dried,'and reweighed. The lossin weight is the water content of the core. The volume occupied by thiswater within the core can be computed from the known salinity of thewater.

Another apparatus suitable for practicing the present invention is thatillustrated in Fig.2.

and porous membrane of fine granular solid l5 corresponding with likeelements described in Fig. 1. In setting up the apparatus, a salineso-'- lution is placed in thistle tube 18 and the membrane I is laiddown in this solution. Excess pressure is then employed to force thesolution down to the upper surface of membrane IS. A suitable means ofapplying the excess pressure is by the use of a stopper l9, clamp 20 andtube 2| connected to a reducing valve 22, through which compressed airis. supplied. The amount of pressure used may be determined from mercuryfilled U-tube 24. The excess pressure is then relieved, as by closing avalve 23 and removing the stopper l9, and sample It is placed inposition on bed IS. The stopper i9 is again put in position and thepressure above membrane l 5 adjusted to the desired value by usingregulation valve 22. The sample is allowed to remain in this apparatusuntil the moisture content of the sample has reached an equilibrium andthe sample is then removed and treated as described in connection withFig. 1.

As stated above, it is preferable to adjust the difference in pressureto correspond with the difference in pressure between the sample inposition in the earth and the height of the water table. It has beenfound, however, that when this difference is equivalent to or exceedsapproximately 30 centimeters of mercury, the employment of a pressuredifferential of 30 centimeters of mercury in analyzing the sample givessubstantially the same results as the pressure difference actuallypresent in the undisturbed reservoir. This is illustrated by Fig. 3.Fig. 3

is a curve showing the manner in which the the density of the reservoirfluids. The water content, after thecore has been allowed to come toequilibrium in this situation, is substantially connate water contentpetroleum reservoir, comprising the steps of.

physically contacting the sample with water, inducing a difference inpressure between the .sample and the water comparable to the pressuredifference between the water table and the connate water in the samplein its original terrestrial position, allowing the moisture content ofthe sample to come to equilibrium, and removing and determining thewater content of the sample.

2. A method of determining connate water content of a sample taken froman underground petroleum reservoir comprising the steps of saturating amembrane with water and retaining it thereafter in at least capillarycontact with a body of water, placing the sample in physical contactwith said membrane, exerting a difierential pressure between saidsample-and said body of water through said membrane comparable to thepressure differential between the connate water of the sample and thewater table in the undisturbed reservoir, and maintaining this pressuredifferential until the moisture content of the sample becomessubstantially constant, and subsequently determining this moisturecontent.

3. A method in accordance with claim 2 in which the pressurediiferential between the connate water and the water table in theundisturbed reservoir is determined as less than 30 cm. of mercury, andthe pressure differential between the sample and the body of water islent pressure differential is more than 30 cm.

mercury, these cores can be run by the present technique without preciseknowledge of the elevation of the water table in the reservoir. Thisfact increases the utility of the present method.

From the facts set forth in the foregoing paragraph, coupled withconventional thermodynamic reasoning, it can be shown that for corestaken from well above the water table it is also unnecessary that thesurface tension of the water equal the interfacial tension of the waterand petroleum in the reservoir. This fact further increasesthe utilityof the method. The phrase well above the water table usually implies adistance of more than 15 to 40 feet vertically. The exact distanceisvariable and depends on the texture of the reservoir rock.

From the foregoing description, it can be seen that the presentinvention comprises essentially bringing the core sample intocommunication with water having a surface tension approximately equal tothe interfacial tension of the water and petroleum in the reservoir,which water in the test exists under a pressure less than that of thesurroundings of the core in the test by an amount which is computablefrom the position of the core within the reservoir and maintained atsubstantially this determined value.

4. A method in accordance with claim 2 in which the pressuredifferential between the connate water and the water table in theundisturbed reservoir is greater than 30 cm. of mercury, and thepressure differential between the sample and the body of water ismaintained at 30 cm. of mercury.

5. A method of determining connate water content of a sample takenfrom'an underground petroleum reservoir comprising the steps ofsaturating a membrane with a saline solution of the same salinity of thewater of the well from which the sample was taken, maintaining capillarycontact between a body of the saline solution and said membrane, placingsaid sample in contact with the membrane on'the opposite side of saidmembrane from the body of solution, adJusting the pressure across themembrane to a constant value so that when equilibrium is establishedbetween the sample and the saline solution the moisture content of thesample will correspond to that of the sample when the sample was in theundisturbed reservoir, and determining said moisture content.

6. The method in accordance with claim 2 in which the surface tension ofthe water comprising said body is substantially equal to the interfacialtension between the water and petroleum within the reservoir.

MILES C. LEVEREI'I'.

